The present invention relates to protective shields for radars and other sensors used in conjunction with guided airframes, and more particularly, to a radome useful with guided missiles and constructed to minimize guidance errors otherwise resulting from thermal expansion of the radome during flight.
A high speed guided missile employing an electromagnetic or other sensor for guidance requires a radome to cover the sensor. Typically, the radome is a conical or rounded hollow shell which encloses the sensor and provides the aerodynamically streamlined forward surface of the airframe. The radome must be transparent to radiant energy in the operative frequency range of the sensor. In addition, the radome must be rigid and heat resistant to withstand the rigors of high speed flight. Radomes have heretofore been constructed from a wide variety of materials such as ceramic material sold under the trademark PYROCERAM 9606 material.
Guidance errors are induced by refraction of the radiant energy as it passes through the radome into the interior thereof. These errors can be minimized by designing for a specific radome thickness. However, during high speed flight, the radome is heated as a result of the friction of the air passing over the radome at high speed and other sources of heat. Because the radome material has a temperature coefficient of expansion, a compromise must be obtained by adjusting the thickness of the radome for an average value over the expected temperature range of the radome in flight.
There are missile systems which use two or more configurations which, in turn, yield two or more ranges of temperatures during flight. In one configuration of interest, the radome temperatures range from about 390.degree. F. to about 620.degree. F. during flight, while in another configuration of interest, the same radome experiences a temperature range of only 250.degree. F. to about 360.degree. F. In a third configuration of interest, the same radome experiences a temperature range of about 360.degree. F. to about 940.degree. F. It would be desirable to use the same radome on the missile in all three configurations, however, this forces a performance compromise on the missile due to differing amounts of thermal expansion of the radome, and thus differing amounts of radiant energy refraction.
U.S. Pat. No. 3,001,473 discloses a rocket nose cone having multiple layers which burn away successively during re-entry into the earth's atmosphere to protect instruments or explosives at the forward end of the missile from excessive heat.
U.S. Pat. No. 3,292,544 discloses a radome having a layered or sandwiched configuration to provide low weight and/or wide frequency bandwith.
U.S. Pat. No. 3,762,666 discloses a radome having a solid cone tip coated with a ceramic or ablative material to divert air and foreign particles outwardly and prevent excessive heating or erosion of the remaining uncoated portion of the radome.
U.S. Pat. No. 3,925,783 discloses a tailored radome with variable thickness layers to minimize refractive distortion.
U.S. Pat. No. 4,173,187 discloses a multi-layer missile re-entry nose cone made of fused silica filled with radiation absorbing particles.
Also of general interest in this field are U.S. Pat. Nos. 2,281,637; 2,854,668; 2,962,717; 3,002,190; 3,063,654; 3,080,816; 3,195,138; 3,301,624; 3,302,884; 3,596,604; and 4,186,900.